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Pluto's pros and cons

Young et al. / SwRI / NASA

This view of Pluto was
created by seeing how its
brightness changed during
partial eclipses by Charon.


To be or not to be ... a planet? That's not really the big question anymore, no matter how you feel about Pluto's so-called demotion.

The truly big question, addressed in two books that look at Pluto's present position from completely different perspectives, has to do with what kinds of planets are out there. That applies to our own solar system as well as the hundreds of other worlds being detected in the universe beyond.

Both books take Pluto as the starting point: In "The Pluto Files," astrophysicist Neil deGrasse Tyson chronicles the rise and fall of the little world's fortunes - a saga in which he played a role as director of the Hayden Planetarium at New York's American Museum of Natural History.

Tyson got himself in trouble with Pluto-loving third-graders when he decided to leave Pluto out of a parade of planets displayed in the museum's Hall of the Universe. This was back in 2000, well before the International Astronomical Union's decision in 2006 to define the word "planet" in such a way that Pluto didn't qualify.

Today, Tyson insists the omission wasn't meant as a snub. "That exhibit was not an exhibit about the solar system," he said. Instead, it was meant to show the relative sizes of cosmic objects ... objects that just happened to include planets. He pointed out that Pluto and its cousins in the Kuiper Belt, a ring of icy objects beyond Neptune, are included in another exhibit that delves into the solar system's full menagerie.

"I think Pluto's happier there, with the kings of the Kuiper Belt," Tyson told me.

So would Tyson say the solar system has eight planets, or nine? "I say the question doesn't interest me," he answered. "The question should not be how many planets there are. There was never any science in that question."

Rather, the question is how similar objects should be grouped together in the solar system: The way most astronomers see it, the four terrestrial planets belong together (Mercury, Venus, Earth and Mars). Then there's the asteroid belt, with Ceres in the leading role. Then there's the four gas giants (Jupiter, Saturn, Uranus and Neptune). Finally, you have the icy bodies on the solar system's edge - which include Pluto as well as Eris, the even farther-flung object that astronomers found to be bigger than Pluto.

"When you take that approach, you end up with a family photo of the solar system," Tyson said, rather than a list that has to be memorized.

W.W. Norton
Hayden Planetarium director Neil
deGrasse Tyson addresses the
planetary debate in "The Pluto Files."


The controversy over what to call Pluto (dwarf planet? distant minor planet? large Kuiper Belt object? comet in a deep freeze?) is just one part of "The Pluto Files," Tyson pointed out. "The rest of the book is a celebration of people's reactions to this scientific controversy, and it's a fun celebration at that," he said.

What is it about Pluto that makes it, as Tyson says in the book's subtitle, "America's Favorite Planet"?

"It's the dog," Tyson said. "It's entirely the dog."

In the book, Tyson delves into the connection with Pluto the Pup, the Disney cartoon character that was named after the solar system sensation in 1930-31. He also reprints editorial cartoons about Pluto's plight. You'll even find reproductions of some of the handwritten letters from kids who took Tyson to task - or took his side.

"I know how you feel," one 8-year-old told Tyson in a note. "We feel the same about Pluto not being a planet. ...  But we just have to get over it. That's science."

'Is Pluto a Planet?'
Unlike Tyson's 8-year-old correspondent, Vanderbilt University astronomer David Weintraub doesn't think the controversy is over - and he doesn't think the way the IAU handled the issue two years ago serves as a good example of how science is done.

"I don't think we have a consensus right now, and science is built on that consensus," he told me.

Princeton Univ. Press
Vanderbilt astronomer David A.
Weintraub has updated "Is Pluto a
Planet?" with a postscript about the
planethood debate.


Weintraub's book on the subject, "Is Pluto a Planet?," takes a longer view on the big question, starting with the ancient Greeks and moving through the history of planetary discovery. At various times through the centuries, astronomers counted 16 planets, or 13, or eight or nine.

Now that scientists have detected more than 300 planets beyond our solar system, plus Pluto-sized "planet embryos" and other oddities, defining planethood per se is looking more and more like a lost cause, Weintraub said. That's the approach he takes in the classroom as well as in the book, which has just been updated in paperback to address the "flawed logic" behind the IAU's rulings on dwarf planets and plutoids

"The word 'planet' by itself doesn't give us enough information to think critically about what someone is telling us," he said. "Jovian ... Neptune-sized ... Earth-sized ... You almost have to have those adjectives in order to make the word 'planet' useful anymore."

So where does Pluto fit in the grand scheme of things? Weintraub said the ultimate answer will depend on several factors - including the growing body of knowledge about extrasolar planets as well as the results from the New Horizons mission to Pluto, which reaches its climax in 2015.

"Come back in 10 years and we'll have a better answer," he said. "I think we're asking better questions right now."

Tale of a telescope
This week, Pluto will be a cause for celebration at the Rancho Hidalgo housing development in Animas, N.M. The developers bought the 16-inch telescope that Pluto discoverer Clyde Tombaugh used in his latter years, and at 3 p.m. MT Wednesday the instrument will be dedicated at its new place of honor.

Courtesy of Gene Turner
An archived photo shows the late astronomer
Clyde Tombaugh, discoverer of Pluto, with his 16-
inch telescope at his home in Las Cruces, N.M.
Click on the picture for a larger version.


The developers for Rancho Hidalgo also own the Arizona Sky Village, another residential community designed to cater to skywatchers. One of the developers, Gene Turner, told me that Tombaugh's telescope will serve as the centerpiece for a new educational program at Rancho Hidalgo. Among those invited to the dedication are members of Tombaugh's family as well as David Levy, Tombaugh's biographer and the co-discoverer of Comet Shoemaker-Levy 9.

Turner expects the refurbished and upgraded telescope to get frequent use. "There's no shortage of people who can usher people up to the eyepiece," he said. Turner himself has his heart set on imaging Pluto and its biggest moon, Charon, as separate objects - something that's rarely been done by amateur astronomers.

He promised that Rancho Hidalgo will be a place where a discouraging word about Pluto will seldom be heard. "If you come in here, Pluto's a planet," Turner told me, "and I will put that in my covenants and restrictions."


You can watch Tombaugh show off his telescopes in video clips that are part of this Q&A with the late astronomer.